It is known to provide air conditioning installations which include a unit for treatment and distribution of air, in which the unit comprises an upstream chamber containing a motorised fan unit and a downstream chamber having distribution valves, the upstream and downstream chambers being separated from each other by an evaporator. A collector for recovering condensate is located below the evaporator. The collector terminates in an outlet which communicates with the outside of the vehicle in which the installation is fitted. The evaporator is arranged to cool a stream of air, which is driven towards the evaporator by the fan of the motorised fan unit and passes from the evaporator to be distributed, under the control of the distribution valves, into the cabin of the vehicle.
In passing through the evaporator, at least some of the humidity which is contained in the blown air stream is extracted from the latter, and collects in the form of condensates in the condensate collector. These condensates are then either treated in various ways known to the person skilled in this art, or they are evacuated outside the vehicle, or both. However, after the installation is turned off, some of the condensate still remains within the air treatment and distribution unit, and undergoes chemical or biochemical changes which tend to render the air which is stagnant within the treatment and distribution unit noxious. As a result, when operation of the air conditioning installation is eventually restarted, the noxious air is conveyed into the cabin of the vehicle, to the discomfort of the occupants.